1. Field of the Invention
The present invention relates generally to circuits with voltage and current sensing and, more specifically, the present invention relates to current and voltage sensing in circuits with voltage drop.
2. Background Information
Most battery operated portable electronic products such as cell phones, personal digital assistants (PDAs), etc. require a low power alternating current (AC) to direct current (DC) charger power supply with a constant voltage and constant current (CC/CV) characteristics for charging batteries. Most of these chargers require relatively accurate and expensive circuitry to meet the specified current and voltage tolerances over temperature.
In a known circuit, voltage is sensed by using an accurate programmable reference IC such as a TL431, which drives an opto-coupler feedback circuit to control the output voltage at a programmed value set by external resistors. A relatively high level of accuracy is needed at the output of the charger circuit in order to meet a lower accuracy at the load due to voltage drop in the output cable that connects the charger to the load (the electronic product). The voltage drop in the output cable reduces the output voltage as load increases degrading the overall voltage tolerance at the load. A required level of voltage accuracy at the charger output can be achieved by choosing a TL431 IC that has been trimmed to the appropriate accuracy level. TL431s with 3%, 2% and 1% accuracy are widely available. TL431 voltage reference is generally more expensive than a simple zener reference. However, zeners are generally difficult to get at tolerance below 2% and the zener voltage varies with the current through it, resulting in a poorer load regulation in circuits that have zener currents that vary with output load due to low gain of the feedback loop. In addition, they are only available in certain standard voltage values, which makes it difficult to center the output voltage at the optimum point for the best tolerance.
For low cost, the current sensing in low power applications (e.g. <5W) is usually done by using a voltage drop across a current sense resistor to turn on a bipolar a transistor. This circuit uses the base emitter voltage, VBE of the transistor as a reference. The transistor in turn drives an optocoupler feedback circuit to control the output current at a constant value. The constant current limit set by such a circuit, however, has a large temperature variation due to the high temperature coefficient of VBE (−2 mV/° C.). This can be compensated to the first order by using a thermister based resistor network which add to the component count and cost.